In several applications, silicon (Si) based power semiconductors are, in the meantime, in the range of their physical limits. Composite semiconductor materials, such as silicon carbide (SiC), provide a remedy.
In SiC-based components, the gate oxide generally has a slight band offset in the conduction band from comparable silicon components, which means that degradation due to tunnel currents already occurs at lower gate field strengths. For SiC transistors, in particular, MOSFET's (metal oxide semiconductor field effect transistor), a sensible field strength in the gate oxide is 3 MV/cm. Adherence to this limiting value is critical, in particular, in cutoff operation and makes design measures necessary, in particular, in the case of trench devices.
The present state of science or the art is the double-trench SiC device design of Rohm from the year 2011, which, with 0.79 mOhm*cm2, attains a top value for the specific forward resistance Ron in the voltage class of 600 V.
The field strength at the gate oxide may be reduced, for example, by introducing a double trench having deep p-implantation. In this context, the p-type areas that are situated deeper constitute a JFET (barrier layer field effect transistor), which shields the actual trench MOSFET structure.
The field strengths at the gate oxide may also be reduced to approximately 4 MV/cm by introducing p-doped regions (p-bubbles) below the gate oxide (J. Tan et al, High-Voltage Accumulation-Layer, UMOSFET's in 4H—SiC, IEEE ELECTRON DEVICE LETTERS, VOL. 19, NO. 12, DECEMBER, 1998).
Alternatively, the two measures mentioned above (double trench, p-bubbles) may be combined (Source 4: Shinsuke Harada et al., “Determination of optimum structure of 4H—SiC Trench MOSFET,” Proceedings of the 2012 24th International Symposium on Power Semiconductor Devices and IC's, pp. 253ff).
The conventional measures for reducing the gate oxide field strength are sometimes only effective to a limited extent or have considerable disadvantages, such as higher required surface area, electrical resistance, and/or process complexity. In particular, the double trench structure requires considerable space, since the requisite trench structure is situated next to the trench gate. Consequently, the specific sheet resistance Ron*A increases and limits the technological progress due to higher integration density. Besides the increased surface requirement, the resistance in parts of the current path is also markedly increased due to the JFET effect of the double trench.